Beryllium is a commonly used material employed as an acceptor to produce p-type semiconductors. For example, beryllium as a p-type dopant may be used to produce p-type III-V semiconductors such as, but not limited to, p-type gallium arsenide (pGaAs), p-type indium gallium arsenide (pInGaAs) and p-type aluminum gallium arsenide (pAlGaAs). P-type III-V semiconductors may be used as a base layer for an NPN heterojunction bipolar transistor (HBT), for example.
However while common, beryllium doping may present certain disadvantages in practice. For example, beryllium is known to diffuse within a semiconductor lattice, especially when under stress due to an electrical bias at elevated temperature. Moreover, while a beryllium-doped semiconductor layer used in a base layer of an NPN HBT may yield relatively high current gain and good carrier mobility, the beryllium-doped semiconductor layer may also exhibit relatively low carrier concentrations, relatively high resistivity and in some instances, a high base-emitter turn-on voltage (VBE) due to beryllium diffusion, for example. In addition, a frequency performance of a device (e.g., HBT) may be relatively poor when beryllium doping is employed.
Carbon is often cited as an attractive alternative to beryllium for the p-type doping of III-V semiconductors. For example, carbon may be more stable than beryllium as a dopant since carbon atoms are large and thus are less likely than beryllium to suffer from diffusion. As such, it is often possible to achieve higher doping concentrations with carbon leading to a relatively lower sheet resistance and a better frequency performance than is generally possible with beryllium doping. Furthermore, owing largely to a lower diffusibility of carbon compared to beryllium, carbon doping may provide better VBE control as well as lower collector-emitter offset voltage (VCE offset) than beryllium doping when the doped III-V semiconductor is used as a base-layer of a transistor (e.g., an HBT). However, carbon as a dopant has a relatively large carrier (i.e., hole) effective mass that may limit carrier (i.e., hole) mobility. Furthermore, a potential exists for hydrogen passivation of carbon atoms when using carbon as a p-type dopant, especially when employing gas source molecular beam epitaxy (GSMBE) to grow and deposit relatively thin layers of the carbon-doped III-V semiconductor.